Subsidence-driven environmental change in three Holocene embayments of Ahuriri Inlet, Hikurangi Subduction Margin, New Zealand

Abstract

Three paleo-embayments on the southwest side of the uplifted Ahuriri Inlet, Hawkes Bay, contain complex interfingering sequences of Holocene terrestrial and saltmarsh peat and intertidal shelly sand and mud. We use 295 foraminiferal samples from 45 short cores (up to 8 14;m deep) to reconstruct the paleoenvironmental history of the bays. We infer that the strongest influence on their paleoenvironmental history was 8-10 14;m of net tectonic subsidence since 7.3 14;ka, which provided the accommodation space for sediment deposition prior to the c. 1.5 14;m of uplift in the AD 1931 Hawkes Bay Earthquake. Much of the subsidence appears to have occurred in eight large co-seismic events (0.4-2 14;kyr recurrence time) which caused marine transgressions of varying amounts into each bay. Sea-level changes (eustatic and isostatic) have been a secondary driver over the middle-late Holocene with an interval (c. 2.6-1.7 14;ka) of widespread erosion or slow sedimentation, caused by a sea-level fall of c. 1 14;m following the middle Holocene highstand. The interval of fastest sedimentation and maximum marine transgression occurred within the last 1 14;kyr and was driven by increased tectonic subsidence, rising sea level and enhanced compaction of thicker sequences of Holocene sediment, particularly peat. The supply of fluvial mud and cliff-eroded sand was the main, relatively constant source of clastic sediment. Airfall and reworking of three pumiceous tephras had a minimal impact on the paleoenvironments of the bays.